Systems and methods for linear accelerator radiotherapy with magnetic resonance imaging

ABSTRACT

Systems and methods for the delivery of linear accelerator radiotherapy in conjunction with magnetic resonance imaging in which components of a linear accelerator may be placed in shielding containers around a gantry, may be connected with RF waveguides, and may employ various systems and methods for magnetic and radio frequency shielding.

RELATED APPLICATION(S)

This application is a continuation of and claims priority to and thebenefit of U.S. application Ser. No. 15/268,366 filed Sep. 16, 2016,titled “Systems And Methods For Linear Accelerator Radiotherapy WithMagnetic Resonance Imaging,” which is a continuation of U.S. applicationSer. No. 13/841,478 filed Mar. 15, 2013, titled “Systems And Methods ForLinear Accelerator Radiotherapy With Magnetic Resonance Imaging,” thedisclosures of each are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to systems and methods for the deliveryof linear accelerator radiotherapy in conjunction with magneticresonance imaging.

BACKGROUND

It is desirable to combine radiation therapy with repetitive real-timeimaging using a magnetic resonance imaging system (MRI) in order tobetter locate and treat therapy targets while sparing nearby healthytissue. While MRIs and radiation treatment systems such as linearaccelerators (linacs) have operated separately from one another forquite some time, combining the two technologies presents manysignificant technological challenges. Such challenges include themagnetic fields and eddy currents generated in ferromagnetic andconductive radiotherapy equipment through the MRI's main magnet andgradient coils, both of which can destroy an MRI's ability to providequality images. In addition, an MRI's main magnetic field can interferewith many of the components of a linear accelerator, and the high-powerradiofrequency (RF) generated by linac components can likewise destroythe imaging capabilities of the MRI.

SUMMARY

Disclosed herein are systems and methods for combining radiotherapy withmagnetic resonance imaging. One embodiment of a disclosed system mayinclude a magnetic resonance imaging system, a gantry, two or moreshielding containers attached to the gantry, at least two of the two ormore shielding containers containing components of a linear accelerator,and at least one radio frequency waveguide connecting componentscontained in at least two of the two or more shielding containers.

In another embodiment, the at least one radio-frequency waveguide isoriented to be substantially perpendicular to the magnetic field linesof a main magnet of the magnetic resonance imaging system and/or mayinclude a magnetic shield.

In a further embodiment the two or more shielding containers are spacedsubstantially equidistant from one another around the circumference ofthe gantry.

In yet another embodiment, the system may include at least one shieldingcontainer that does not contain components of a linear accelerator.

In one embodiment, the system may include multiple radio frequencywaveguides, the radio frequency waveguides extending substantiallyaround the entire circumference of the gantry. In some cases, at leastone of the radio frequency waveguides will not transmit radiofrequencywaves.

In certain embodiments, the system may include three shieldingcontainers adapted to contain RF power source components in a firstshielding container, circulator and AFC components in a second shieldingcontainer, and linear accelerator components in a third shieldingcontainer.

In yet another embodiment, the at least one radio frequency waveguideincludes RF shielding. The RF shielding may be an RF absorbing material,an RF reflecting material, or multiple layers of RF reflecting andabsorbing materials. And may include carbon fiber, silicon carbide,copper, aluminum, or copper or aluminum alloys or oxides.

In some embodiments, the RF shielding may include water cooling or aircooling.

Embodiments of the disclosure may also include a method of providing amagnetic resonance imaging system, providing a gantry, affixing two ormore shielding containers to the gantry, placing components of a linearaccelerator into at least two of the two or more shielding containers,and connecting components contained in at least two of the two or moreshielding containers with at least one radio frequency waveguide.

In some embodiments of the method, the at least one radio-frequencywaveguide may be oriented to be substantially perpendicular to themagnetic field lines of a main magnet of the magnetic resonance imagingsystem. In other embodiments, the at least one radiofrequency waveguidemay include a magnetic shield.

In other embodiments of the method, multiple radio frequency waveguidesmay be included, extending substantially around the entire circumferenceof the gantry. In some embodiments, the at least one radio frequencywaveguide may include RF shielding, which may be an RF absorbingmaterial, an RF reflecting material, or multiple layers of RF reflectingand absorbing materials and may include air or water cooling.

These and other features, aspects, and advantages of the presentdisclosure will become better understood with reference to the followingdescription and claims.

BRIEF DESCRIPTION OF DRAWINGS

Features, aspects, and implementations of the disclosure are describedin conjunction with the attached drawings, in which:

FIG. 1 is a simplified diagram illustrating aspects of a radiationtherapy device operating in conjunction with a magnetic resonanceimaging system consistent with implementations of the current subjectmatter;

FIG. 2 is a top view of the device depicted in FIG. 1;

FIG. 3 is a simplified diagram illustrating various components of anexemplary linear accelerator;

FIG. 4 is a section view of the gantry and associated components of theexemplary device depicted in FIG. 1;

FIG. 5 illustrates a map of magnetic field strength around an exemplaryMRI, consistent with implementations of the current subject matter.

DETAILED DESCRIPTION

Disclosed herein are systems and methods for combining radiotherapy withmagnetic resonance imaging. FIG. 1 is a simplified schematic view of anexample radiation therapy system including shield containers 104 mountedon a gantry 106, which can rotate to different positions to enableradiation delivery from different angles. The exemplary system depictedin FIG. 1 also includes an MRI 102, which may be used for real-timeimaging during radiation therapy and maybe of the split or open type ofMRI as shown (as is beneficial so the radiation beam need not betransmitted through the side of an MRI). Radiation therapy devices maybe placed inside shield containers 104 and can be used to direct atreatment beam at a target within patient 108 lying on couch 110. Alsodepicted are waveguides 112, which may be used to connect radiationtherapy device components, as explained further below. FIG. 2 depicts atop view of the simplified example system shown in FIG. 1. A similarsystem is described in U.S. Pat. No. 8,190,233 to Dempsey, titled“System for Delivering Conformal Radiation Therapy while SimultaneouslyImaging Soft Tissue,” which is hereby incorporated by reference. Thesystem of the present disclosure differs in many respects from thatdisclosed in Dempsey '233, a primary difference being that the radiationtherapy system of the present disclosure specifically includes a linac.

Magnetic resonance imaging is primarily a medical imaging technique mostcommonly used in radiology to visualize the internal structure andfunction of the body. MRI is described, for example, by E. MARK HAACKEET AL., MAGNETIC RESONANCE IMAGING: PHYSICAL PRINCIPLES AND SEQUENCEDESIGN (Wiley-Liss 1999), which is hereby incorporated by reference. Thesplit magnet system illustrated in FIG. 1 includes a pair of mainmagnets and can also include conventional MRI components not shown, suchas a split gradient coil, shim coils and an RF system. The strength ofthe magnetic field generated by the main magnets can vary, but in anembodiment of the disclosed system the main magnet field strength is0.35 T.

A linear particle accelerator (also called a linac) is a type ofparticle accelerator used to accelerate subatomic ions at great speeds.Linacs are described, for example, by C. J. KARZMARK ET AL., MEDICALELECTRON ACCELERATORS (McGraw-Hill, Inc., Health Professions Division1993), which is hereby incorporated by reference. The linac may bedesigned to accelerate electrons of relatively low energy, in the rangeof 4 to 6 mega-electron volts (MV) accelerating potential, and have astanding wave guide to keep it compact and, for example, may operate atS-band or X-Band frequencies.

FIG. 3 includes a simplified diagram of some of the main components of alinear accelerator 300. The simplified, exemplary linac may include apulse modulator 304 that may amplify AC power from supply 302, rectifyit to DC power, and produce high-voltage DC pulses used to powerelectron gun 312 and RF power source 306. High-voltage cableselectrically connect pulse modulator 304 to the electron gun 312 and RFpower source 306. RF power source 306 may be, for example, a magnetronor klystron.

RF power source 306 produces and sends pulses of microwaves, with pulsepowers that may be approximately 2.5 mega watts (MW), to acceleratingresonating cavity 316 through a waveguide 307. Waveguides 307 may bepressurized by a waveguide gas system 308. Accelerating resonatingcavity 316 may be evacuated by vacuum pump 318 and utilize RF pulsesfrom RF power source 306 to accelerate an electron beam 314 generated byelectron gun 312. Electron gun 312 creates bursts of electrons, whichenter accelerating resonating cavity 316, a resonating cavity excited byRF pulses from the RF power source 306, accelerating an electron beam tonear the speed of light.

Electron beam 314 may optionally be aimed at a target 320, commonly madefrom tungsten, to produce Bremsstrahlung X-rays for x-ray/photon-beamtherapy, or the target may be removed for electron-beam therapy. Theresulting beam may optionally pass through flattening filter 322 in head324 prior to entering collimator 326, which may be a multi-leafcollimator as described further below.

The exemplary, simplified linear accelerator 300 depicted in FIG. 3 alsoincludes a circulator 310 and an automatic frequency control system(AFC) 328. The circulator 310 can control the flow of RF waves. Forexample, it can send energy reflected by the waveguide to an RF dumpinstead of allowing it back to RF power source 306, which could causeinterference or damage. Circulator 310 may also communicate reflected RFwaves to AFC 328, which may monitor the reflected waves to determinewhether the resonant frequency of accelerating resonating cavity 316 haschanged, for example, due to heating. AFC 328 may then communicate withcontrol unit 332, or directly with RF power source 306, to adjust thefrequency of the RF waves emitted by RF power source 306.

In one embodiment of the disclosure, various components of a linearaccelerator, including, but not limited to the linac componentsillustrated in FIG. 3, may be separated into two or more sets ofcomponents that may be attached to gantry 106. FIG. 4 depicts oneembodiment of such an arrangement, in which linear acceleratorcomponents may be grouped and placed within shield containers 104 aroundgantry 106. Where the particular groupings of linear acceleratorcomponents require as much, RF waveguides 112 can be placed aroundgantry 106, connecting the various shield containers 104 and the linearaccelerator components within. For example, an RF waveguide 112 would berequired if RF power source 306 was located within one shield containerand the linear accelerator, including electron gun 312, acceleratingresonating cavity 316, target 320 and head 324, was in a separate shieldcontainer 104 (as shown in FIG. 3, a waveguide 307 is required totransmit the RF energy from the RF power source 306 to the acceleratingresonating cavity 316).

This disclosure contemplates any number of potential divisions orgroupings of linear accelerator components, and any number of shieldcontainers 104 spaced around gantry 106 to contain such components. Inaddition, the components to create more than one linear acceleratorcould be divided and grouped in shield containers 104 around gantry 106if multiple radiation therapy beams were desired.

In one embodiment, depicted in FIG. 4, there may be three shieldcontainers 104 spaced substantially equidistant from one another aroundthe circumference of gantry 106, with waveguides 112 connecting theshield containers 104 in series. Various groupings of linear acceleratorcomponents may be placed in each of shielding containers 104. In anexemplary embodiment, the major components of a linear accelerator maybe divided as such: RF power source components 404 may be placed withinone shield container 104, circulator and AFC components 406 may beplaced in another shield container 104, and linear acceleratorcomponents 402 (e.g., electron gun 312, accelerating resonating cavity316, target 320, head 324 and collimating device 326) may be placed in athird shield container 104. In this embodiment, as well as in othercontemplated embodiments, additional linear accelerator components maybe distributed amongst the shield containers 104 as is convenient. Inaddition, certain linear accelerator components may be located off ofgantry 106. For example, pulse modulator 304 may be located on thegantry, on the gantry supporting stand, in a separate cabinet outsidegantry 106, or possibly outside the RF shielding room of the system. Thesystems and methods of the disclosure do not require any particularnumber of shielding containers 104 or any particular groupings orlocations of linear accelerator components. The embodiments describedherein are merely examples consistent with aspects related to thedescribed subject matter, and any limitations on particular arrangementsmay only be made in the claims.

One embodiment of the present disclosure may include one or moreshielding containers 104 as described herein, or shielding containers104 consisting merely of materials that mimic the ferromagnetic andconductive aspects of the shielding containers, placed around gantry106, which do not contain components of a linear accelerator. Suchadditional shielding containers 104 may be included when shieldingcontainers are not required to hold and/or shield linear acceleratorcomponents, but are beneficial in simplifying the ability to shim theoverall system for the magnetic field homogeneity necessary for qualityimaging by MM 102. Similarly, embodiments of the disclosure may includeone or more waveguides 112, which may merely be made of similarmaterials that mimic the ferromagnetic and conductive aspects of otherwaveguides 112, when it is not necessary to transmit RF waves from oneshielding container 104 to another (because the container does notinclude linear accelerator components or because the components withinthe shielding container do not involve the transmission of RF waves).

Embodiments of shield containers 104 have been described in U.S. patentapplication Ser. No. 12/837,309 to Shvartsman et al., entitled “MethodAnd Apparatus For Shielding A Linear Accelerator And A MagneticResonance Imaging Device From Each Other,” which is hereby incorporatedby reference. Shield containers 104 can be designed to shield variouslinear accelerator components from the magnetic field of MRI 102. Oneexample of such a shield includes a shell made of high magneticpermeability material. The shell may be cylindrical in shape with one orboth ends of the shell being open. While a cylindrical shape ispreferred, the disclosed shield shells can be other shapes. The shellcan have a thickness chosen according to characteristics of the shellmaterial and magnetic field being shielded against. The shell may beformed of non-oriented silicon steel, for example a nickel-iron alloy,such as the commercially-available material sold by ThyssenKrupp Steelunder the trade name 530-50 AP and having a thickness of, for example,about 5 mm. Other material options include M19 steel, M45 steel, andCarpenter High Permeability “49” Steel. The outer diameter and length ofthe shell can vary; in the one embodiment, the outer diameter is about30 cm and the length is about 70 cm.

In some embodiments, shield container 104 can include multiple shieldshells. The multiple shield shells may be concentric/coaxial layers ofsteel, which can be separated by layers of air or other insulatingmaterial. In such embodiments, the inner shell(s) can be of a higherpermeability but a lower saturation flux density than the outer shells,as the outer shell has already greatly reduced the magnetic field fromthe MM 102. In another embodiment, a current carrying coil may be usedinside of the inner shell or outside of an outer shell to cancel theresidual field.

Embodiments of shield containers 104 may also contain RF shielding todecrease the leakage of RF energy from linear accelerator components tothe surroundings. Such shielding may take the form of additional shellsof RF absorbing and/or RF reflecting material, as detailed inapplication Ser. No. 12/837,309, and further below.

As discussed above, radiofrequency waveguides 112 are structures thatcan transmit RF wave energy, for example, from RF power source 306 tocirculator 310 and accelerating resonating cavity 316. In theembodiments of the disclosure, it is contemplated that at least onewaveguide 112 will connect two shield containers 104 containingcomponents of a linear accelerator. In other embodiments, waveguides 112will connect multiple pairs of shield containers 104. In an exemplaryembodiment, waveguides 112 will connect each of the multiple shieldcontainers 104 located on gantry 106, spanning substantially around theentire circumference of gantry 106. As detailed above, such anembodiment may be implemented even if the linear accelerator componentscontained in each of the shield containers 104 do not need to beconnected by a waveguide. This embodiment may be beneficial tofacilitate shimming of MRI 102 for optimal magnetic field homogeneity.

In one embodiment, waveguides 112 may extend from RF power sourcecomponents 404 to circulator and AFC components 406 to linearaccelerator components 402, and back to RF power source components 404,as depicted in FIG. 4. If necessary, multiple RF waveguides 112 mayextend between shield containers 104. For example, if it was necessaryto transmit RF waves in both directions between two shield containers104 based upon the linear accelerator components contained within. Insuch an embodiment, the same number of waveguides would preferably beplaced between each pair of shield containers 104 so that substantialsymmetry would exist around the entire circumference of gantry 106.

In one embodiment, waveguides 112 may be made from copper. In otherembodiments waveguides 112 may be formed from multiple materials, suchas, a non-ferromagnetic metal coated on the interior with copper,silver, gold or another conductive metal. In an exemplary embodiment,waveguides 112 may be pressurized by waveguide gas system 308 with aninert gas such as SF-6 to prevent dielectric breakdown and may have thefollowing specifications: Hollow Rectangular Waveguide, EIA: WR284,RCSC: WG10, IEC: R32, S band, Recommended Frequency Band (GHz):2.60-3.95, Lower Cutoff Frequency GHz 2.078, Higher Cutoff Frequency GHz4.156, Inner Wave Guide Dimensions (Inches): 2.840×1.340, with wallthickness WG10: 0.08 inches. Waveguides 112 should also be designedwithin bending radii restrictions, as is known in the art.

In embodiments where waveguides 112 are not required to transmit RFwaves, they may merely be made of materials that mimic the ferromagneticand conductive qualities of the other waveguides 112.

In one embodiment, waveguides 112 may be magnetically shielded,utilizing, for example, the concepts, materials and designs discussedabove with respect to shielding containers 104. Shielding concepts anddesigns that may also be used are disclosed in U.S. patent applicationSer. No. 13/801,680 to Shvartsman et al. entitled “Systems And MethodsFor Radiotherapy With Magnetic Resonance Imaging,” which is incorporatedherein by reference.

In an exemplary embodiment, waveguides 112 do not require magneticshielding, but instead are oriented to be substantially perpendicular tothe main MRI magnet's field lines. FIG. 5 shows an exemplary map ofcontours 502 of magnetic field strength magnitude for a split MRI 102having a 0.35 T main magnet. FIG. 5 depicts a top view of an exemplarymain magnet 504 as is shown in FIG. 2 also, as magnet halves 102. Rightangle indicator 506 in the contour map of FIG. 5 shows that the magneticfield lines for MRI 102 at the preferable location of waveguides 112, asalso shown in FIG. 2, will result in the waveguides 112 beingsubstantially perpendicular to the magnetic field lines.

The systems and methods of the present disclosure also include numeroustypes and placement of radiofrequency shielding and absorbing materials.As discussed above, a linear accelerator's RF power source 306 andelectron gun 312 involve the generation of significant radiofrequencyenergy. Such energy is also transmitted throughout the system viawaveguides and within additional linear accelerator components such as acirculator 310 or an AFC 328. Embodiments of radiofrequency shieldingdisclosed herein control the dispersion and transmission of such RFenergy, so as to limit the negative effects on the MRI's ability toacquire quality images resulting from eddy currents or interference withthe MRI's radio frequency coils.

As noted above, one embodiment of the disclosure involves the inclusionof RF shielding as part of shield containers 104 in the form of, forexample, one or more shells of RF reflective materials such as copper oraluminum, and/or RF absorbing materials such as carbon fiber or SiliconCarbide (SiC). Embodiments may include any number of layers. In someembodiments, the layers of shells can be made of combinations ofdifferent materials or of the same material. For example, in someembodiments, the shield shell layers can include alternating layersformed of RF absorbing material and RF reflecting material. In suchembodiments, it is desirable to provide an air gap between the layers ofshield shells.

When shield containers 104 include linear accelerator componentsinvolving the substantial generation of RF energy, the containers 104may also optionally include RF shielding that covers the top and thebottom of an open cylindrically shaped shield. When shield container 104contains the linear accelerator itself, the treatment beam will thuspass through the RF shielding. In such a case, the RF shield material ispreferably uniform and minimally attenuating to the radiotherapy beam.

In addition to the inclusion of RF shielding in conjunction with shieldcontainers 104, certain embodiments of the disclosure will provideadditional RF shielding, for example, around the waveguides 112. Theamount of RF energy leakage from the waveguides 112 is likely to besmall, and will not necessarily require shielding, however, RF shieldingis preferably included at any points where leakage is more likely, suchas at flanges, connection points for the waveguides, RF sinks, couplers,etc.

Shielding material may include RF absorbing material such as carbonfiber or silicon carbide (SIC) and/or RF reflecting material such ascopper or aluminum. In some embodiments it may be advantageous toprovide a number of alternating layers of RF reflecting material and RFabsorbing material.

Additionally, such RF absorbing/shielding material can be used to linethe interior surface of the room in which the system of the currentdisclosure is placed. The room walls, ceiling and floor could be linedwith meshed or chopped carbon fiber, carbon fiber wallpaper, carbonfiber panels, carbon fiber paint, etc. Furthermore, RFabsorbing/shielding material may be placed on the outer surfaces of MRI102, on gantry 106, and on any linear accelerator components or othercomponents not placed on gantry 106 or MRI 102 (for example, if pulsemodulator 304 is not located on the gantry).

The RF shielding materials disclosed herein may be flexible and wrappedaround the various components, or may be molded to fit the shape of thecomponents.

Cooling can be provided as needed to the RF shielding/absorbingmaterials. A variety of known cooling methods can be used. The coolingsystem may include, for example, fluid-carrying conduit for circulatinga fluid in the vicinity of the shielding/absorbing materials. Also,air-cooling can be provided by incorporating a system for moving airacross the surfaces of the RF shielding/absorbing materials.

FIG. 4 also depicts collimating devices 408 attached to gantry 106 andassociated with each of the shield containers 104. Collimating devices408 may, for example, be multi-leaf collimators (MLCs), which typicallyhave two banks of opposing pairs of leaves that move independently andcan open to form apertures of various shapes and sizes. The leaves maybe made of tungsten or any suitable material or materials for blockingradiation. MLCs may also employ a tongue and groove arrangement on thelong sides and front of the leaves to limit interleaf radiation leakageand can be configured for inter-digitation of the leaves in the closedposition. Each leaf of each bank of leaves may be capable of independentmotion and may be driven by leaf motors through connecting rods. An MLCcontrol system can control the two opposing banks of leaves toindependently position the edge of each leaf to a specified location inorder to block a radiation beam and form a field size of a specificshape. The MLC leaves, motors, and other components may be supported byhousing that then attaches to gantry 106. The housing may be, forexample, made from aluminum.

In one embodiment, there may be only one collimating device 408,associated with the one shield container 104 that contains the linearaccelerator components creating the actual treatment beam. Theadditional devices 408 may simply be additional conductive elements,utilized to reduce the negative imaging effects of eddy currentsgenerated during operation of the MRI's gradient coils. Such devices canbe designed as described in U.S. patent application Ser. No. 13/801,680to Shvartsman et al. entitled “Systems And Methods For Radiotherapy WithMagnetic Resonance Imaging,” which is incorporated herein by reference.Consistent with the teachings of the incorporated application,embodiments of the system of this disclosure may include, for example,additional conductive elements. In one embodiment, a multi-leafcollimator occupies the space shown as 408 adjacent the shield container104 containing the linear accelerator, while five additional equallyspaced conductor elements are attached around the remainingcircumference of gantry 106.

Additional shimming and shielding concepts disclosed in application Ser.Nos. 13/801,680 and 12/837,309 are also applicable to the systems andmethods disclosed herein, and are also fully incorporated by reference.For example, additional shimming for magnetic field homogeneity can beprovided by permanent magnets, optionally made from Neodymium (NdFeB).The polar orientation of the permanent magnets should be such that theycounteract the MRI's main magnetic field and the magnetic field inducedin various ferromagnetic materials in the vicinity of MRI 102. Thecanceling effect of the magnet design's strengths, field orientations,and locations can be determined utilizing modeling software such asFARADAY, available from Integrated Engineering Software, or any otherappropriate software such as VectorField, for example, and with furtheranalysis of results potentially being performed in a program such asMATLAB or any other appropriate software such as FORTRAN, for example.As an alternative to permanent magnets, active windings could also beused. Such active shimming concepts are disclosed in U.S. applicationSer. No. 13/324,850 to Shvartsman et al. entitled “Active ResistiveShimming For MRI Devices,” also incorporated herein by reference.

The subject matter described herein can be embodied in systems,apparatus, methods, and/or articles depending on the desiredconfiguration. The implementations set forth in the foregoingdescription do not represent all implementations consistent with thesubject matter described herein. Instead, they are merely some examplesconsistent with aspects related to the described subject matter.Although a few variations have been described in detail above, othermodifications or additions are possible. In particular, further featuresand/or variations can be provided in addition to those set forth herein.For example, the implementations described above can be directed tovarious combinations and subcombinations of the disclosed featuresand/or combinations and subcombinations of several further featuresdisclosed above. Other implementations may be within the scope of thefollowing claims.

What is claimed is:
 1. A system for radiotherapy, the system comprising:a gantry; a first component of a linear accelerator attached to thegantry; a second component of the linear accelerator attached to thegantry at a separate location around the gantry; a radio-frequency (RF)waveguide connecting at least two components of the linear acceleratorattached to the gantry; a first RF shielding around the first componentof the linear accelerator configured to decrease leakage of RF energyfrom the first component of the linear accelerator to surroundings; anda second RF shielding around the second component of the linearaccelerator configured to decrease leakage of RF energy from the secondcomponent of the linear accelerator to the surroundings.
 2. The systemof claim 1, wherein the first RF shielding includes a first shell andthe second RF shielding includes a second shell.
 3. The system of claim2, wherein the first shell contains RF power source components, and thesecond shell contains an accelerating resonating cavity.
 4. The systemof claim 3, further comprising a third shell containing a circulator. 5.The system of claim 1, wherein the first RF shielding or the second RFshielding comprises at least one of: an RF absorbing material, an RFreflecting material, and multiple layers of RF reflecting and absorbingmaterials.
 6. The system of claim 1, wherein the first RF shielding orthe second RF shielding comprises at least one of copper, aluminum,alloys of copper and aluminum, oxides of copper and aluminum, carbonfiber, and silicon carbide.
 7. The system of claim 1, wherein the firstRF shielding or the second RF shielding comprises at least one of watercooling and air cooling.
 8. The system of claim 1, further comprising amagnetic resonance imaging system.
 9. The system of claim 8, furthercomprising one or more shielding containers attached at separatelocations around the gantry and configured to shield one or morecomponents of the linear accelerator from a magnetic field of themagnetic resonance imaging system.
 10. The system of claim 9, whereinthe one or more shielding containers are spaced substantiallyequidistant from one another around a circumference of the gantry. 11.The system of claim 10, wherein the one or more shielding containerscomprises at least one shielding container that does not containcomponents of the linear accelerator.